Support metric.metadata in pdata/pmetric (#10006)

#### Description

After
#9985, add
support for the new metric.metadata field in the pmetric package.

I plan to use the metric.metadata field to support additional prometheus
types per
https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/compatibility/prometheus_and_openmetrics.md#metric-metadata

.chloggen/metric-metadata.yaml

@@ -0,0 +1,25 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. otlpreceiver)
+component: pmetric
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Support metric.metadata in pdata/pmetric
+
+# One or more tracking issues or pull requests related to the change
+issues: [10006]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: [api]

pdata/internal/cmd/pdatagen/internal/pmetric_package.go

@@ -115,6 +115,10 @@ var metric = &messageValueStruct{
 			defaultVal: `""`,
 			testVal:    `"1"`,
 		},
+		&sliceField{
+			fieldName:   "Metadata",
+			returnSlice: mapStruct,
+		},
 		&oneOfField{
 			typeName:                   "MetricType",
 			originFieldName:            "Data",